The invention relates to a light emitting diode for use with an optical apparatus. Also, the invention relates to an optical apparatus for determining an optical property of a sample.
Conventionally, in the field of laboratory examination, there has been used a variety of optical apparatuses or analyzers which comprises a light source for emitting light onto a sample to be analyzed, and a light detector for receiving the light reflected from (or transmitted through) the sample in order to determine the reflectance (or transmittancy) of the sample. Further, in a small sized analyzer, a light emitting diode having a PN junction semiconductor has been preferably employed for the light source.
FIG. 6 illustrates an optical system which has been used for such an analyzer. The analyzer includes a light source or light emitting diode (LED) 10 for emitting a light Q1. The emitted light Q1 is illuminated on a test sample (not shown) on a plate 11. Then, a reflected light Q2 from the sample is detected by an optical sensor 12. The sensor 12 transmits a signal having a voltage corresponding to an intensity of the reflected light to obtain the reflectance of the sample on the plate 11.
Disadvantageously and undeniably, a luminous efficiency of the LED tends to vary depending upon an environmental temperature and electric current (voltage) to be applied and to decrease in proportion to its service time. Also, the change of luminous efficiency will adversely provide the measured result with error. Therefore, it is important to determine the intensity of light emitted from the LED at measurement and, if necessary, the resultant measurement should be corrected based upon the change of light efficiency.
One method for determining the change of light intensity of the emitted light, which is shown in FIG. 6, is to dispose a beam splitter 13 or a half-mirror across the light Q1 so that the light Q1 is divided into a measuring beam Q1' for illumination of the sample and a monitoring beam Q3 for detecting the light intensity of the light Q1 by the use of an optical sensor 14. Another method, which is shown in FIG. 7, is to dispose an optical sensor 15 beside LED 10 so that a feeble light Q3' emitted laterally from the light source 16 is detected, thereby determining the intensity of the light Q1.
The first method, however, renders the analyzing system more complicated and larger in size because of the additional arrangement of the beam splitter 13 or half mirror between the LED 10 and the test sample.
The second method has another drawback that the intensity of feeble light emitted laterally does not directly reflect the intensity of light Q1. Specifically, FIG. 8 is a graph showing a direction dependency of the light intensity of the LED. In this graph, a curve G1 illustrates a direction dependency in each direction against the light intensity of light Q1 having the maximum light intensity, in which the intensity of light emitted in the lateral direction spaced at 90 degrees from the upright direction is only about 30% of that of the upright direction. Further, another curve G2 also illustrates a direction dependency when the light intensity in the upright direction is reduced to 50% by decreasing the voltage (current) applied to LED in which the light intensity of lateral direction is only about 10%, not 30%, of that of the upright direction in curve G1.
More disadvantageously, the light intensity varies with the environmental temperature as well as the emitting direction. This is illustrated in a three dimensional graph of FIG. 9 in which X, Y and Z axes represent an angle of each direction from the upright direction when the upright direction is zero degree, environmental temperature (from 5 to 50 degrees centigrade) and light intensity in each direction, respectively. In this graph, the light intensity in each direction is standardized at 5 degrees centigrade. This shows that the intensity of light changes with the environmental temperature, but the increasing rate in one direction differs from that of another direction. Namely, with the increase of the environmental temperature, the ratio of light intensity between the monitoring light Q3' and the primary light Q1 varies depending upon the emitting angle.
It is believed that the direction dependency of the light intensity is caused by a variation of density of the current which flows in the light source, that may occur from differences of electric properties of assembled devices, connecting condition of electrodes of the light emitting diode, and the like.
Thus, the light intensity of the primary light Q1 emitted in the normal direction could not be determined precisely by the detection of the monitoring light Q3' emitted laterally, inhibiting the precise measurement of the optical property of the sample.
Accordingly, it is an object of the invention to provide a light emitting diode which is preferably used in the optical apparatus.
It is another object of the invention to provide an optical apparatus for use in an optical analyzer.
It is still another object of the invention to provide an optical apparatus capable of measuring an optical property such as reflectance or transmittancy of the test sample with a great precision.